PSCustomObject does not work with Select-Object -Unique and Sort-Object -Unique

Prerequisites

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Steps to reproduce

Given any collection of PSCustomObjects with different fields, they will not compare equal but still collapse into a single item under Select-Object -Unique or Sort-Object -Unique. Notable cmdlets outputting PSCustomObject collections are:

• Select-Object itself (-Unique works when used immediately, but not later on)
• ConvertFrom-Csv

This behavior has been mentioned in https://github.com/PowerShell/PowerShell/issues/15806 and https://github.com/PowerShell/PowerShell/issues/12059 , but neither are for this exact problem, and it affecting Select-Object transformations and ConvertFrom-Csv makes it very prominent.

Expected behavior

> $files = Get-ChildItem | Select-Object Name

> $files
Name
----
dotnet-sdk-5.0.408-linux-arm64
dotnet-sdk-6.0.400-linux-arm64

> $files[0] -eq $files[1]
False

> $files | Select-Object -Unique
Name
----
dotnet-sdk-5.0.408-linux-arm64
dotnet-sdk-6.0.400-linux-arm64

Actual behavior

> $files | Select-Object -Unique
Name
dotnet-sdk-5.0.408-linux-arm64

Error details

No response

Environment data

Name                           Value
----                           -----
PSVersion                      7.3.0-preview.3
PSEdition                      Core
GitCommitId                    7.3.0-preview.3-304-gd02c59addc24e13da3b8ee5e1a8e7aa27e00c745
OS                             Linux 5.15.0-1013-raspi #15-Ubuntu SMP PREEMPT Mon Aug 8 06:33:06 UTC 2022
Platform                       Unix
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0…}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0

Visuals

No response

[SteveL-MSFT]

@PowerShell/wg-powershell-cmdlets reviewed this and agree that the current behavior for PSCustomObject is incorrect. We believe this is likely a bucket 3 breaking change that users are relying on the current behavior. We considered an option to inform the user to use -Property * if we detect that the objects are PSCustomObjects, however, it seems more useful to fix this behavior that future users can rely upon. So the change is only if the first object is a PSCustomObject and -Property isn’t specified, then it gets set to -Property *.

[iSazonov]

Looking at the results of the proposed fix, I am not satisfied. While the main scenario is fixed by this fix (usually all objects of the same type in the pipeline), I’m afraid that we’ll immediately get feedback that it doesn’t work if the first object isn’t a PSCustomObject. Since we’re doing a slow accumulation of objects from the pipeline anyway, there’s nothing stopping us from checking their type until we encounter a PSCustomObject.

This in turn makes me think that this fix is more of a workaround and the main problem is somewhere deeper and should be fixed, although it’s not trivial anymore.